The object of this project is to explore the physiological significance of (leu 5)-enkephalin (LE) in neuronal function. Both (met 5)-enkephalin (ME) and (LE) are stored in neurons and are unevenly distributed in brain. Whether these two peptides exist in the same, or different, neurons still remains unclear. In order to study the role of these two peptides we have developed a specific and sensitive method for the LE assay. The method is simple and can be used for measuring leu-enkephalin in crude tissue extract. A study on the distribution of LE and ME in brain regions of various species indicates that a high concentration between the two enkephalins was also observed in drug-treated rats. Chronic treatment of rats with haloperidol or lithium increases the LE and ME content in striatum and N. accumbens but not in other brain areas. ME and LE are also present in sympathetic ganglia, again deafferentation of the ganglia increases both peptides. The results fail to oppose the view that these two enkephalins may be stored in the same neurons.